1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus including the fixing device.
2. Description of the Related Art
In an image forming apparatus such as a copier or a printer, a fixing device taking a dielectric heating is known for reducing a start-up time of the fixing device.
Such a dielectric heating fixing device includes a conveyance belt that conveys a recording medium with an unfixed toner image thereon and a plurality of pairs of rod electrodes located facing the recording medium conveyed by the conveyance belt. By applying a high-frequency electricity to the rod electrodes, heat is generated in the unfixed toner on the recording medium by dielectric loss, i.e., the toner image is heated by dielectric heating. The melted toner penetrates the fibers of the recording medium and is thereby fixed onto the recording medium.
The dielectric heating fixing device may be used with an additional fixing device that uses a heater. The additional fixing device is located on the downstream side relative to the dielectric heating fixing device in a direction of movement of the recording medium and includes a fixing roller with the heater therein and a pressure roller that presses against the fixing roller to form a nip portion therebetween. In this system, after the toner image is primarily fixed onto the recording medium by dielectric heating as described above, the recording medium is conveyed to the nip portion in the additional fixing device. The toner image is securely fixed onto the recording medium by application of heat and pressure in the nip portion.
There is another type of fixing device that uses electromagnetic induction heating instead of the dielectric heating to reduce the start-up time of the fixing device. Such a fixing device generates an alternating magnetic field around a heat generation member, thereby generating heat in the heat generation member by eddy current or hysteresis loss. A fixing member can be configured as the heat generation member to be heated by electromagnetic induction.
With regard to the above-described dielectric heating fixing device, as noted above the toner melted by dielectric heating penetrates the fibers of the recording medium, thereby fixing the toner image onto the recording medium. However, such fixation may be insufficient, particularly in forming a color image, since a gap generated between color toners in the fixed image may result in insufficient gloss.
By contrast, sufficient fixation can be ensured by using an additional fixing device with a heater as described above, since an image is primarily fixed by dielectric heating and secondarily fixed by application of heat and pressure in the nip portion formed between the rollers. However, this arrangement, which requires the additional fixing device with a heater in addition to a dielectric heating fixing device, may increase the device size, cost, and power consumption. Further, the effect of reduction in the start-up time resulted from dielectric heating may not be satisfactory.
The fixing device using electromagnetic induction heating does not directly solve the problems described above, since the configuration and the heating mechanism thereof are different from those of the dielectric heating fixing device.